Liquid crystal displays are of two general types. One employs a light scattering effect, generally known as dynamic scattering, while the other employs polarized light with the liquid crystal characterized by twist nematic properties. Both types of displays can be used as low power devices by use of a reflector panel placed behind the display so that ambient light or a lower power light source can be used to illuminate the display.
In a light scattering display, the liquid crystal material is initially transparent. When activated by applying an electric field, the material is converted to a milky white state. This is caused by the creation of scattering centers in the liquid. When the voltage is removed, the material returns to its transparent state.
Liquid crystal displays of the dynamic scattering mode are activated to maximum scattering by current on the order of 1 to 5 .times. 10.sup.-6 amperes in a typical dynamic scattering liquid crystal display of 10.mu. thickness to consume 10.sup.-4 to 10.sup.-5 watts/cm.sup.2 of display area. This current and power is of sufficient intensity to deteriorate the liquid crystal. This occurs long prior to erosion of the electrodes. Therefore, to extend the life of such systems, emphasis has been placed upon extending the life of the liquid crystal itself.
Certain work has been performed to increase the electrical conductivity of liquid crystals in the dynamic scattering mode. In an article at pages 24 and 25 of the Japanese magazine, "Nikkei Electronics", dated Apr. 22, 1974, the addition of equimolar quantities of hydroquinone and parabenzoquinone is disclosed to increase the conductivity of a dynamic scattering mode display device upon application of an electrical field. The article includes experimental data which indicates that the hydroquinone and parabenzoquinone should be used only in equimolar combination to provide complex ions in the liquid crystal for this purpose.
In the field effect type of liquid crystal display, a twist nematic material is placed in a thin film between transparent electrodes in the form of a 90.degree. twist from the inner surface of one electrode to the inner surface of the other one. Unpolarized light is linearly polarized in the Y-direction by the first polarizer and follows the 90.degree. twist as it passes through the liquid crystal and arrives properly oriented in the X-direction parallel to the second polarizer. Light is then reflected back through the liquid crystal material following the 90.degree. twist, arriving aligned in a Y-direction and will pass out through the first polarizer. To an observer, the display looks clear bright. When the electric field is applied across the two electrodes, the liquid crystal molecules align themselves with the field so that unpolarized light passes through the field aligned liquid crystal film without changing polarization, and is absorbed by the X-direction polarizer. The display looks black due to the absence of reflected light. When the electric field is removed, the liquid crystal resumes its 90.degree. twist configuration. In an alternative embodiment, if the polarizers are oriented parallel to each other, the display appears black without an electric field and clear bright when an electric field is applied.
In contrast to the conventional dynamic scattering displays, a relatively low current, e.g., 10.sup.-7 to 10.sup.-8 amperes, is required to untwist a field effect liquid crystal at 10.mu. thickness. Also, less power is consumed, e.g., 10.sup.-6 to 10.sup.-7 watts/cm.sup.2. It has been found that at such low current and power consumption, the limiting factor on the life of the display device is the electrode life rather than the liquid crystal life.